Function Parameters vs. Prologue (fastcall + O1 + -fschedule-insns2)

[Geyslan Gregório Bem]

Hi,

I know that -fschedule-insns2 reorder the instructions to avoid
bubble, but I did a benchmark that resulted in a more fast code
without this reordering.

What do you have to tell me?

Tks.

Geyslan Gregório Bem
hackingbits.com
@geyslangb
br.linkedin.com/in/geyslan


2013/6/18 Geyslan Gregório Bem <geyslan@gmail.com>:
> Hi,
>
> I'm curious about what is causing this reordering (prologue with the
> function parameters):
>
> # gcc structs.c -o structs -m32 -O1 -fschedule-insns2
>
> (gdb) disas main
> Dump of assembler code for function main:
>    0x0804868c <+0>: push   %ebp
>    0x0804868d <+1>: mov    $0x3,%edx
>    0x08048692 <+6>: mov    %esp,%ebp
>    0x08048694 <+8>: mov    $0x2,%ecx
>    0x08048699 <+13>: and    $0xfffffff0,%esp
>    0x0804869c <+16>: call   0x804845c <funcao>
>
>
> The code:
>
> __attribute__((fastcall)) void funcao(int i, int a) {
> ...
> int main() {
>
>         int i, a;
>         i = 2;
>         i = 3;
>
>         funcao(i, a);
> ...
>
>
> When I remove one of them (fastcall or schedule-insns2), the binary is
> generated in logic order:
>
> (gdb) disas main
> Dump of assembler code for function main:
>    0x08048340 <+0>: push   %ebp
>    0x08048341 <+1>: mov    %esp,%ebp
>    0x08048343 <+3>: and    $0xfffffff0,%esp
>    0x08048346 <+6>: sub    $0x10,%esp
>    0x08048349 <+9>: movl   $0x3,0x4(%esp)
>    0x08048351 <+17>: movl   $0x2,(%esp)
>    0x08048358 <+24>: call   0x8048450 <funcao>
>
> ... or
>
> (gdb) disas main
> Dump of assembler code for function main:
>    0x0804868c <+0>: push   %ebp
>    0x0804868d <+1>: mov    %esp,%ebp
>    0x0804868f <+3>: and    $0xfffffff0,%esp
>    0x08048692 <+6>: mov    $0x3,%edx
>    0x08048697 <+11>: mov    $0x2,%ecx
>    0x0804869c <+16>: call   0x804845c <funcao>
>
> Someone knows something about the combined use of the fastcall
> attribute and schedule-insns2 option? Is there an issue?
>
> Regards.
>
> Geyslan Gregório Bem
>
> http://hackingbits.com
> @geyslangb
> br.linkedin.com/in/geyslan